Features and Benefits

Measurements International (MI), in keeping with “Metrology is our Science, Accuracy is our Business™”, has introduced the first commercially available Automated Dual Source High Resistance Bridge (model 6600A). The Dual Source High Resistance Bridge principle has become the primary measurement technique of all national measurement laboratories (NMIs) around the world due to its unprecedented level of accuracy and stability over various other methods. Multiple modes of operation in the 6600A allows users to make resistance measurements in different ways.

Bridge Mode

Using the Bridge mode, the UUT is measured live against a standard reference resistor where the ratio of the sources is equal to the ratio of resistances (E1/E2 = R1/R2). This is the most accurate and reliable type of measurement for high resistance. Lead Set N-type to N-type cables are supplied.

Direct Mode and Insulation Resistance Mode

The Direct and Insulation Resistance modes which is made on the R1 (shown in Figure 1) channel allow users to perform teraohmmeter type measurements without the need of connecting a reference standard resistor where R = E1/I1. These modes are useful to determine the nominal value of the resistor, its settle time, leakage resistance, etc. N-type to N-type lead and N-type to alligator lead are supplied.

All modes of operation offer users the flexibility to choose UUT test voltages from 1 V to 1000 V with six digits of resolution. Other commercially available units are limited within this range and do not allow the full freedom that the MI 6600A can offer.

Metrologists in any laboratory now have the ability to achieve measurement results and uncertainties that were previously only achievable in the highest of NMI laboratories for measurements above 100 MΩ. This is due to the advanced technology of the Dual Source Bridge. While other commercially available “metres” are limited to only measure one artifact at a time, the 6600A offers an actual “true bridge” measurement mode to complete this process where a known standard is measured against an unknown resistor in “true bridge” technology form. Ratios of 1:1, 10:1, 100:1 or even 1000:1 can be made with the best uncertainties being measured at ratio 1:1 or 10:1 at any voltage.

Ratio Measurement Principle (Figure 1)

The Bridge architecture in the 6600A is formed by substituting two of the resistive arms of a Wheatstone bridge circuit with low-impedance digitally-programmable voltage sources.

http://mintl.com/wp-content/uploads/6650_DualSource-300x153.png

Figure 1. Bridge Block Diagram

Voltage Sources

http://mintl.com/wp-content/uploads/SourceCalibrationExample-300x233.jpg

Figure 2. Source Calibration Example

The 1000 V DC sources in the 6600A offer voltage ratios of 1:1, 10:1, 100:1 and 1000:1 or any ratio in between. The voltage settings can be observed on the front panel display. The model 6600A is the only commercially available high resistance bridge which offers any selectable voltage between 1 V to 1000 V. Other manufacturers are limited to specific voltages within the set range of 1 V to 1000 V. The 6600A is housed in a shielded chamber to limit the effect of the outside environment due to air movement and static electricity caused by people passing by or technicians working in the area.

6600A Calibration 

Calibration of the 6600A is achieved by calibrating the sources and the electrometer. A Diagnostic Menu included in the software allows the user to run diagnostics on all the units in the system. A built-in calibration routine which controls an external calibrated 3458A DMM calibrates the sources one at a time. Source voltages are measured and stored automatically in the source table as Source 1 and Source 2 as offsets from nominal. Figure 2 shows an example of Source 1 calibration. These offsets (Delta) are applied to the voltages selected during measurement. As an alternative with better accuracy, the MI model 8000B system can be used to calibrate the sources.

http://mintl.com/wp-content/uploads/ElectrometerCalibration-300x233.jpg

Figure 3. Electrometer Calibration

Electrometer calibration is achieved by using a calibrated source and calibrated resistor to determine the absolute current readings on the electrometer (Figure 3 shows electrometer calibration example). The effect of the uncertainty on the electrometer is reduced greatly when the 6600A is operated in the bridge mode as the electrometer is used over a very small window reducing the type B uncertainty to effectively zero leaving only the type A uncertainty. In the direct mode the type B uncertainty combined with the type A uncertainty is used.

Leakage Paths

In the 6600A, the resistors are measured with the case grounded, so that a leakage resistance path between the high terminal of the resistor and case is effectively in parallel with the voltage source A, and does not contribute to any measurement errors. Terminal leakage resistance at the low resistor terminal is effectively in parallel with the detector where a very small voltage is present to drive this leakage.

Connections and Thermal Voltages 

The resistors are connected to R1 and R2. Either can be designated as the standard. The reversal method is used to eliminate thermal voltage signals and to allow the capacitance effect of the leads and the resistor time constant to settle.

Software

Making a measurement on the 6600A is simple, software allows the user to define the measurement and test parameters utilizing the same proven software used on the MI 6010 and 6000 series of DC Bridges that our customers have become accustomed to. Features include resistor protection, settle time to verify time constants of resistors, number of measurements, statistics, voltage, temperature coefficients and graphing.

Programs Menu allows users to create and combine several program tasks such as intercomparing standards to standards, standards to measurands, and measurands to measurands. Selectable parameters include measurement time delays (wait times), reversal rate, number of measurements, number of stats, applied voltage, standards used, current threshold, and standard resistor uncertainty. All measurement parameters including statistical data is displayed during measurement.

Measurement Analysis

The 6600A software allows the user to fully analyze all measurements. In the Measurements screen, the measurement data includes the ratio, absolute value and the measurement graph. Voltage and Power graphing allows the user to observe a measurement trend for the current measurement or after the measurement is complete. All the measurement data can be exported to Excel using the supplied Excel Macro. There, further analysis can be performed with the ability to crop out sample sets or remove outliers.

Resistor data can be stored in the history file based on serial number which is updated every time the resistor is measured. In the Resistor History file menu, the resistor data can be recalled and graphed including the standard deviation of each measurement. A trend line based on linear, logarithmic or polynomial waveshapes can be displayed. Regression analysis can also be used to determine projected values over time and the Y axis can be scaled to fit the graph.

Voltage Extension

An optional model 66001 Voltage Extender is available to increase the E1 output of the 6600A to 5 kV for direct mode measurements. This is useful for the calibration of high-voltage standard resistors used to verify hipot or insulation breakdown testers or surface and volume resistivity measurements.